Our long-term goal is to understand the molecular events necessary for mitochondrial gene expression and the nuclear/cytoplasmic interactions required for mitochondrial biogenesis. Using yeast as an experimental organism allows biochemical, molecular biological and genetic approaches to this complex process. Our focus is on tRNAs which are coded by mitochondrial DNA. They play an indispensable role in mitochondrial gene expression and their biosynthesis requires both nuclear and mitochondrial encoded components. Aim 1 is to complete the isolation of the nuclear genes that code for mitochondrial tRNA biosynthetic enzymes so their primary structures, their expression and their relationship to analogous enzymes that function in the nucleus/cytoplasmic compartments can be understood. A mitochondrial specific RNase P consisting of nuclear coded protein and a mitochondrial coded RNA offers unique opportunities to understand the structure-function relationships of RNA requiring enzymes and the nucleo/cytoplasmic interactions necessary for assembly of ribonucleoprotein complexes in mitochondria. Aim 2 is to use the size variation of this RNA in different yeasts to define a functional core of the mitochondrial RNAs and to relate this core to eubacterial RNase P RNA. Aim 3 is to use a novel transformation system to assess the effects that mutations made in vitro have on RNase P in vivo. Aim 4 is to use a combination of molecular biological, biochemical and genetic approaches to describe the pathway of RNase P assembly in mitochondria. Basic scientific investigations such as those proposed here are as essential as applied and clinical investigations in increasing our ability to make progress in solving biomedical problems. RNA enzymes have potential medical applications yet our understanding of how they work is incomplete. Novel findings arising from studying mitochondrial genes have, in the past and will in the future, continue to contribute insights to all aspects of gene expression. Finally, mitochondria play a central role in cellular metabolism and defects in mitochondrial DNA as well as in nuclear genes required for mitochondrial biogenesis are known to be the underlying cause of an increasing number of inherited mitochondrial myopathies. Thus, an understanding of the nucleo- cytoplasmic interactions required for mitochondrial biogenesis will help to understand, and hopefully treat, human disease.